Saw this linked on another site, thought I would share. Some info/book(let)s on historical dying techniques. I am quite tempted to buy the series just because it looks interesting!
Historical Dyes Series (http://www.imperial-purple.com/books.html)
The author is a member of Medieval Dress and Textile Society (http://www.medats.org.uk/). Not much info on their site but might be an interesting membership, nonetheless.
Neat! There is a lady at Pennsic each year (except last year!) who creates her own dyes using what was available in the 12th-14th centuries in Northern Europe. I've talked to her a ton of times and she is amazing at what she does. Here are a picture of her work:
http://www.flickr.com/photos/jubileel/3502736500/
She uses both linen and wool threads to dye given that those were what was available in the period. What she has discovered, and what I have discovered through her work is, if you see it in nature, no matter how odd a color it is, it can be made into a dye. (One day, as I passed by her, I was wearing a very bright pink dress that some of my friends teased me about. So, in front of one of the worst offenders, I asked if this shade of pink was period. Turns out, if you boil saffron with linen, you get that exact shade. The dress was linen so...I used a safforn dye. ;-) )
EDIT: I'm glad someone found the photo above of value. Please, when you link it to your facebook account, could you warn me ahead of time?
I took a course on natural dyes through my city's leisure guide...I should track down the book and share pieces of it with you - it was fairly basic, but covered a fair number of dyes and mordants as well, so you'd probably enjoy it.